MONTPELIER — State mental health employees could begin working at a new residential treatment facility next week with the first patients arriving by the end of the month, about four months later than first expected, officials said Tuesday.

The seven-bed Middlesex Secure Residential Program is part of interim changes developed by the Department of Mental Health after flooding from Tropical Storm Irene in 2011 forced the state hospital in Waterbury to be abandoned and its patients placed elsewhere across the state.

The new building is considered temporary. After three years, department officials will evaluate its effectiveness and determine whether to close it or make other arrangements.

Work on the building, on what was a softball field next to the state police barracks just off Interstate 89, began in late November. It was to open in January for people who no longer require acute care but still need treatment within a secure setting for an extended period.

Many issues contributed to delays, and costs increased from about $1 million to about $1.5 million.

Dave Burley, facilities and operations regional director for the Department of Buildings and General Services, said the contractor felt the residence could be built with modular units in six to eight weeks. The arrangement would allow the site to open in January, but the plans weren’t fully developed.

Mental Health Commissioner Mary Moulton said work orders also had to be redone.

“In this case, we have a facility that had to have some additional security measures and some safety measures that didn’t come with the modular units,” she said.

As another example, plans initially called for the installation of electric heat, which is faster and less expensive to install but more expensive to use. Instead, the building has a gas heating system, which in the long run will save the state money, said Buildings and General Services Commissioner Michael Obuchowski.

Another needed step was covering so-called ligature points that patients can use to harm themselves.

If the building passes safety tests May 14, staff can begin working the next day, the facility can be licensed, and patients can arrive about two weeks later.

President and CEO Rick Cochran of MMIC, a St. Johnsbury company that built the residence, said that while job-change orders delayed the opening and increased the cost, his company still saved the state money overall and completed the facility quickly given the circumstances.

“Had this been designed out and set of plans and specs issued, everything we’d done would have been included,” Cochran said. “The reality is we did some things on the front end that in my mind saved the state some money.”